Preheat oven to 400 degrees F. Line muffin tins with 16 paper muffin liners.: The moment you put the batter together, you want the oven to be ready so that the leavening agents respond correctly. You should feel a warm, anticipatory hum in the kitchen as the oven climbs to 400 degrees F , and the scent of rising warmth signals readiness. A properly preheated oven promotes quick rise and those desirable domed tops. If you skip this, muffins may spread instead of rising, resulting in flatter tops. Common mistake to avoid, do not set a lower temperature to make more muffins at once, it alters rise and texture.
In a large bowl whisk egg, yogurt, milk, vanilla, and vegetable oil until smooth.: As you mix, the batter will emulsify into a glossy, cohesive liquid and you can see small ribbons form when you lift the whisk. The aroma of vanilla will bloom and the mixture should be homogenous with no streaks of separated oil. This wet base is the backbone of moisture and flavor, and whisking well ensures the egg and oil are evenly distributed. If the wet ingredients remain lumpy, the flour will not hydrate uniformly and you might end up with dry pockets. Avoid overwhisking, which can incorporate excess air and lead to uneven rise.
Add flour, sugar, chocolate chips, cocoa powder, salt and baking soda.: When the dry ingredients meet the wet, you will notice a thicker, glossy batter beginning to take shape and the cocoa will darken the mixture to a deep, inviting brown. Stir gently until you no longer see streaks of flour, then fold in the mini chocolate chips so they are dispersed. The combination here is crucial, because overmixing will develop gluten and make the crumb tough, while undermixing leaves dry pockets. A common pitfall is adding the chips too early or letting them clump, which causes uneven distribution and sinking in the muffin cup.
Scoop mixture into prepared muffin cups, about 2/3 full in each muffin cup.: As you portion the batter, it should hold its shape and feel spoonable but not runny. The surface will be slightly glossy and studded with chips. Filling each cup to roughly two thirds gives the batter room to rise without spilling over, and you will see the batter dome up into a rounded top while baking. If you overfill, expect overflow and uneven baking. A common error is filling liners unevenly which leads to muffins that bake at different rates.
Bake in preheated oven until a toothpick inserted into the center comes out clean, approximately 15-19 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack.: During the bake the kitchen transforms, with a warm chocolate aroma that intensifies as the muffins color. Tops should develop a slightly firm crust and a subtle crackling sound as steam escapes. The internal crumb will set and the edges will pull slightly away from the liners. Use a clean toothpick to test doneness, and aim for it to emerge with a few moist crumbs but not raw batter. Underbaking yields a gummy center, while overbaking makes them dry. Keep a close eye during the final minutes to avoid overcooking.
Cool in the pans for 10 minutes before removing to cool completely on a wire rack.: After pulling the muffins from the oven, the steam inside needs a few minutes to settle so the structure firms up. Cooling for about ten minutes in the pans allows the interiors to finish setting while the tops remain tender. When transferring to a wire rack , you will feel the faint warmth dissipate and the muffins will stop steaming, reducing sogginess. Removing them too soon can cause them to break apart, and leaving them in the pan too long may trap moisture and make the bottoms soggy.